1. Field of the Invention
The invention relates generally to subsea exploration systems. More particularly, the invention relates to a marine electromagnetic measurement system and a method of using the same. More particularly still, the invention relates to a net-like array of electrodes to be deployed to a sea floor location wherein a portion of the electrodes rest on the sea floor and a portion of the electrodes are buoyant.
2. Background Art
Hydrocarbon exploration typically involves various prospecting methods, including various geophysical methods to detect the presence of hydrocarbons in the natural void space of the rock (measured as “porosity”) or to map structural features in a formation of interest which are capable of trapping hydrocarbons.
To be mapped, the formation containing the hydrocarbons must possess a physical property contrast to which the geophysical method responds. For example, seismic methods involve emitting seismic waves into earth formations and receiving the reflected and/or diffracted seismic waves from the earth formations. Acoustic waves reflect off interfaces between different types of rocks with dissimilar seismic impedances. The velocities of the reflected or diffracted seismic waves depend on the densities of the rocks, which are in part due to the porosities and fluid contents of the rocks. However, the velocities of the reflected or diffracted seismic waves have very low sensitivity to the types of fluid (e.g., water or oil) in the pores, except for the presence of gas. Thus, seismic methods are useful in mapping the interfaces between different types of rocks. However, certain earth formations are not conductive to exploration through seismic methods. Salts, carbonates, and other particular formations may scatter seismic energy when it is propagated therethrough because of large velocity contrasts and inhomogeneities located within those formations.
In contrast, electrical conductivity (s), or its inverse, resistivity (?), is a physical property that can be measured with various electrical or electromagnetic (EM) methods. Such methods include, but are not limited to, direct current (DC) resistivity, induced polarization (IP) resistivity, magnetotelluric (MT) resistivity, and controlled source electromagnetic (CSEM) resistivity measurements. Regardless of the method employed, the measured resistivity of a formation depends strongly on the resistivity of the pore fluid and the porosity of the rock. Typical brine in sedimentary rock is highly conductive. The presence of brine in bulk rock renders the rock conductive. Hydrocarbons are, by comparison, electrically non-conductive. Consequently, the electrical conductivity of a rock is reduced when hydrocarbons are present. In general, different rocks in a given sedimentary section will have different porosities, so even in the absence of hydrocarbons, information about the sedimentary section can be determined. Thus, the combination of seismic and resistivity data is useful in assessing hydrocarbon content.
As mentioned above, one manner in which resistivity of a formation can be measured is through controlled source electromagnetic (CSEM) stimulation. As the name implies, a controlled transmitter stimulates a known current that is made to flow into the formations to be measured. Often, in CSEM systems, a circular loop of wire carrying a time-varying current is used as a controlled magnetic field source. This produces a time-varying magnetic field in the surroundings. The time-varying magnetic field in turn (according to Faraday's Law) produces a voltage which drives currents in the earth subsurface. Those currents produce voltages that are detected by electromagnetic receivers.
Typically, in marine CSEM applications, a high powered transmitter is towed by a surface ship and an array of receivers resting on the seafloor measures the voltages induced thereby. The induced CSEM voltage signals are detected by electrodes included in sensor packages or by a string of electrodes connected to a cable laid on the seafloor. Examples of receiver packages for detecting CSEM signals, for example, is disclosed in U.S. Pat. No. 5,770,945 issued to Constable and U.S. Pat. No. 6,842,006 issued to Conti, et al. Electrodes on a cable may be spaced a large distance apart to increase the sensitivity of electric field measurements. In addition, because the electrode array is constructed in a single cable, deployment is made simpler and require less capital investment.
While the prior art sensor packages and electrode cables for electromagnetic signal measurements in a marine environment are simple and cost effective, there is still a need for other sensor arrays that can provide more convenient measurements of various signals.